Interpretive Summary: Many estrous synchronization programs available to producers have not been documented to be effective in cattle that have not started estrous cycles. We have shown that estrous synchronization using intravaginal progesterone releasing inserts and prostaglandin F2 alpha followed by an injection of estradiol benzoate 24 to 30 h after insert removal increased fertility, even if the cattle had not started estrous cycles at the beginning of treatment. This combined program has the potential to increase the effectiveness of estrous synchronization especially in females without corpus luteum activity, so that artificial insemination and timed insemination can be more easily used in beef cattle.

Technical Abstract:
Beef heifers (n=57) and postpartum cows (n=52) received an IVP4 (EAZI- BREED**TM CIDR**R) for 7 d (d 0 = insert day) with an injection of PGF2alpha on d 6. At 24 to 30 h after IVP4 removal, EB was injected at the following doses: heifers 0, .2, .38, or .75 mg and cows 0, .25, .5, or 1 mg. Percentage of females showing estrus was increased by EB(P<.04) with greatest response at .38 mg in heifers (86%) and 1 mg in cows (100%). Dose x time interaction affected (P<.01) E2 concentrations in heifers and cows with animals receiving the higher doses of EB having greater estradiol (E2) concentrations in a shorter time than those re- ceiving the smaller doses. Percentage of cows and heifers with an acute preovulatory LH release (peak LH) was affected by dose with a linear (P<.01) and quadratic (P<.01, increase was less as dose increased) re- sponse. Highest concentrations of LH during peak LH were affected by dose with a linear (P<.01) response in heifers and linear (P<.01) and quadratic (P<.08, increase was less as dose increased) response in cows. Heifers receiving .38 mg and cows receiving .5 and 1 mg of EB had the highest peak LH. Time to LH peak had a linear(P<.03) response in heifers and linear (P<.04) and quadratic (P<.05) response in cows. Pregnancy rate was affected (P<.02) in heifers by anestrus vs cycling before IVP4 treatment (52% vs 22%) and in cows by dose of EB (P<.01; 8, 23, 21, and 67% for 0,.25,.5, & 1 mg, resp.). In conclusion, in females treated with IVP4 and prostaglandin F2alpha to induce and synchronize estrus, an in- jection of EB increased concentrations of E2 and LH and increased number of animals showing estrus. Also, EB increased pregnancy rates in cows. Optimal responses were at .38 mg EB for heifers and at 1 mg EB for cows.